Manufacturing method of opto-electric hybrid board and opto-electric hybrid board obtained thereby

ABSTRACT

A method of manufacturing an opto-electric hybrid board which is capable of reducing the number of steps for the manufacture of the opto-electric hybrid board and which achieves the reduction in thickness of the opto-electric hybrid board to be manufactured, and an opto-electric hybrid board obtained thereby. A resist layer is formed on a core-forming resin layer, and is then formed into a predetermined pattern. Resultant portions of the core-forming resin layer serve as cores (optical interconnect lines)  3.  Next, a thin metal film  5  is formed on the under cladding layer  2  so as to cover the resist layer and the cores  3.  Thereafter, the resist layer is removed together with portions of the thin metal film  5  lying on the surface of the resist layer. Electroplating is performed on the remaining portions of the thin metal film  5  to fill grooves  6  defined between adjacent ones of the cores  3  with electroplated layers  7   a  obtained by the electroplating. The electroplated layers  7   a  serve as electrical interconnect lines  7.

RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application No.61/028,664, filed Feb. 14, 2008; which is hereby incorporated byreference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method of manufacturing anopto-electric hybrid board in which an optical waveguide and electricalwiring are combined, and to an opto-electric hybrid board obtainedthereby.

2. Description of the Related Art

Recently, information and communications using light as a medium havecome into widespread use. An opto-electric hybrid board in which anoptical waveguide and electrical wiring are combined (see, for example,Japanese Patent Application Laid-Open No.2001-7463) has accordingly beenemployed as a board for use in electronic devices for information andcommunications and the like.

In general, this opto-electric hybrid board is structured such that anelectrical wiring board including electrical interconnect lines(conductors) formed in a predetermined pattern and an optical waveguideincluding cores (optical interconnect lines) formed in a predeterminedpattern and serving as a passageway for light are stacked together. Anexample of the opto-electric hybrid board is shown in FIG. 3. Theopto-electric hybrid board B shown in FIG. 3 has a multi-layer structurehaving two layers in which an optical waveguide β is formed on anelectrical wiring board α. In the above-mentioned electrical wiringboard α, a plurality of electrical interconnect lines 96 are buried inan insulation layer 95 and are also supported by another insulationlayer 94 in that state. In the above-mentioned optical waveguide β, aplurality of cores 93 are buried in an over cladding layer 98 and arealso supported by an under cladding layer 92 in that state.

In the method of manufacturing the above-mentioned conventionalopto-electric hybrid board B, however, the process of producing theoptical waveguide β is performed after the process of producing theelectrical wiring board α, and each of the processes involves the needfor a multiplicity of steps. Thus, it takes a long period of time tomanufacture the opto-electric hybrid board B. For example, thepatterning of the electrical interconnect lines 96 in the electricalwiring board a involves the need for a large number of steps such as thesteps of patterning a resist through exposure, development and the like,plating other portions than the resist, and then removing theabove-mentioned resist. The patterning of the cores 93 in the opticalwaveguide β also involves the need for a large number of steps such asexposure, development and the like.

Additionally, the above-mentioned conventional opto-electric hybridboard B has the two-layer structure in which the optical waveguide β isstacked on top of the electrical wiring board α. Thus, theabove-mentioned conventional opto-electric hybrid board B isdisadvantageous in reducing the thickness thereof, and cannot respond torecent requests for the reduction in thickness.

In view of the foregoing, it is an object of the present invention toprovide a method of manufacturing an opto-electric hybrid board which iscapable of reducing the number of steps for the manufacture of theopto-electric hybrid board and which achieves the reduction in thicknessof the opto-electric hybrid board to be manufactured, and anopto-electric hybrid board obtained thereby.

To accomplish the above-mentioned object, a first aspect of the presentinvention is intended for a method of manufacturing an opto-electrichybrid board, which comprising the steps of: forming a core-formingresin layer on an under cladding layer; forming a resist layer on thecore-forming resin layer; forming the resist layer and the core-formingresin layer into a predetermined pattern by patterning to form thecore-forming resin layer into cores; forming a thin metal film on theunder cladding layer with the resist layer and the cores patterned so asto cover the resist layer and the cores; removing the resist layercovered with a portion of the thin metal film together with the portionof the thin metal film; electroplating the remaining portion of the thinmetal film lying on side surfaces of the cores patterned to protrude andlying on a portion of the under cladding layer between the cores to fillgrooves defined between adjacent ones of the cores with plated layers,thereby causing the plated layers to serve as electrical interconnectlines; and forming an over cladding layer so as to cover the cores andthe electrical interconnect lines.

A second aspect of the present invention is intended for anopto-electric hybrid board obtained by the above-mentioned method ofmanufacturing the opto-electric hybrid board, which comprises: aplurality of protruding cores formed in a predetermined pattern on anunder cladding layer; a thin metal film formed over side surfaces of theprotruding cores and a surface portion of the under cladding layerexcept where the protruding cores are formed; electrical interconnectlines including electroplated layers buried in grooves defined betweenadjacent ones of the protruding cores; and an over cladding layer formedso as to cover the protruding cores and the electrical interconnectlines.

In the method of manufacturing the opto-electric hybrid board accordingto the present invention, the plurality of protruding cores (opticalinterconnect lines) are formed in a predetermined pattern, andthereafter the grooves defined between adjacent ones of the cores arefilled with the plated layers obtained by the electroplating. The platedlayers serve as the electrical interconnect lines (conductors). In otherwords, the electrical interconnect lines are produced by using thegrooves defined between the cores serving as one of the components of anoptical waveguide and the like. Thus, the present invention eliminatesthe need to form a new pattern of the electrical interconnect lines andaccordingly reduces the number of steps for the manufacture of theopto-electric hybrid board. As a result, the present invention iscapable of reducing the time required for the manufacture of theopto-electric hybrid board to achieve improvements in productionefficiency. Further, since the pattern of the electrical interconnectlines is formed by using the pattern of the cores as mentioned above,the positioning accuracy of the cores and the electrical interconnectlines is automatically increased. Additionally, since theabove-mentioned electrical interconnect lines are formed by using thegrooves defined between the cores of the optical waveguide, themanufactured opto-electric hybrid board has what is called asingle-layer structure, and is significantly thinner than theconventional opto-electric hybrid board having a two-layer structure.

When the under cladding layer is formed on the metal base, only theopto-electric hybrid board is easily obtained by removing only theabove-mentioned metal base by etching after the opto-electric hybridboard is manufactured on the metal base.

In particular, when the metal base is a base made of stainless steel,the manufacture of the opto-electric hybrid board on the base isaccomplished with stability because the base made of stainless steel isexcellent in corrosion resistance and in dimensional stability.

The opto-electric hybrid board according to the present invention haswhat is called a single-layer structure because the electricalinterconnect lines are formed by using the grooves defined betweenadjacent ones of the cores as mentioned above. Thus, the above-mentionedcores and the electrical interconnect lines are formed at the samevertical position. This enables the opto-electric hybrid board accordingto the present invention to be significantly thinner than theconventional opto-electric hybrid board having a two-layer structure.

BRIEF DESCRIPTION OF DRAWINGS

FIGS. 1( a) to 1(d) and FIGS. 2( a) to 2(c) are views schematicallyillustrating a method of manufacturing an opto-electric hybrid boardaccording to one preferred embodiment of the present invention; and

FIG. 3 is a view schematically illustrating a conventional opto-electrichybrid board.

DETAILED DESCRIPTION

A preferred embodiment according to the present invention will now bedescribed in detail with reference to the drawings.

FIGS. 1( a) to (d) and FIGS. 2( a) to (c) illustrate a preferredembodiment of a method of manufacturing an opto-electric hybrid boardaccording to the present invention. In this preferred embodiment, anunder cladding layer 2 made of an insulative material is initiallyformed on the entire top surface of a base 1 in the form of a stainlesssteel plate and the like (as shown in FIG. 1( a)). Subsequently, acore-forming resin layer 3 a made of an insulative material is formed onthe under cladding layer 2. Thereafter, a resist layer 4 is formed onthe core-forming resin layer 3 a (as shown in FIG. 1( b)). Next, theabove-mentioned resist layer 4 together with the core-forming resinlayer 3 a is formed into a predetermined pattern (as shown in FIG. 1(c)). Portions of the core-forming resin layer 3 a formed into thepredetermined pattern to protrude serve as cores (optical interconnectlines) 3. Subsequently, a thin metal film 5 is formed on the undercladding layer 2 with the above-mentioned patterned resist layer 4 andcores 3 by sputtering, electroless plating or the like so as to coverthe resist layer 4 and the cores 3 (as shown in FIG. 1( d)). Thereafter,the above-mentioned resist layer 4 is removed by etching (as shown inFIG. 2( a)). In this process, portions of the thin metal film 5 whichare formed on the surface of the resist, layer 4 are also removedtogether with the above-mentioned resist layer 4. The remaining portionsof the thin metal film 5 are formed on a surface portion of the undercladding layer 2 and on opposite side surfaces 32 of the cores 3. Byelectroplating the remaining portions of the thin metal film 5, grooves6 defined between adjacent ones of the above-mentioned cores 3 arefilled with electroplated layers 7 a (as shown in FIG. 2( b)). In thisstate, the electroplated layers 7 a (lying in the grooves 6 definedbetween adjacent ones of the above-mentioned cores 3 serve as electricalinterconnect lines (conductors) 7. Then, an over cladding layer 8 madeof an insulative material is formed to cover the above-mentioned cores 3and the above-mentioned electrical interconnect lines 7 (as shown inFIG. 2( c)). In this manner, an opto-electric hybrid board A having whatis called a single layer structure in which the cores (opticalinterconnect lines) 3 and the electrical interconnect lines (conductors)7 are arranged alternately is provided. This opto-electric hybrid boardA, which is formed on the base 1, is manufactured into a product afterthe base 1 in the form of a stainless steel plate is removed bytreatment with an acid and the like or manufactured into a producttogether with the base 1.

In this opto-electric hybrid board A, the under cladding layer 2, thecores 3 and the over cladding layer 8 constitute an optical waveguide,and the electrical interconnect lines 7 are formed in the grooves 6defined between the cores 3. The electrical interconnect lines 7, whichare formed by utilizing the insulating properties of the under claddinglayer 2, the cores 3 and the over cladding layer 8, are highly reliable.Also, the thin metal film 5 is formed on the opposite side surfaces 32of the cores 3, and may be used to function as a surface for reflectinglight beams. This further ensures the propagation of light beams.

The above will be described in further detail. The base 1 on which theabove-mentioned under cladding layer 2 is formed (as shown in FIG. 1(a)) has a flat shape, and a metal plate such as the above-mentionedstainless steel plate is used for the base 1. The material of the base1, however, is not limited to this. Examples of the material of the base1 used herein may include glass, quartz, silicon, synthetic resins andthe like. The thickness of the base 1 is, for example, in the range of20 μm (for a film-like base 1) to 5 mm (for a plate-like base 1).

The formation of the above-mentioned under cladding layer 2 (as shown inFIG. 1( a)) is carried out, for example, in a manner to be describedbelow. First, a varnish prepared by dissolving a photosensitive resin (ahighly insulative resin) conventionally known in the art in a solvent isapplied onto the above-mentioned base 1, and is then dried by a heatingtreatment to thereby form a photosensitive resin layer. Next, thephotosensitive resin layer is exposed to irradiation light, and aheating treatment is performed on the exposed photosensitive resinlayer, whereby a photoreaction is completed. This forms theabove-mentioned photosensitive resin layer into the under cladding layer2. The thickness of the under cladding layer 2 (the photosensitive resinlayer) is typically in the range of 10 to 1000 μm.

For the formation of the above-mentioned under cladding layer 2, theapplication of the above-mentioned varnish is achieved, for example, bya spin coating method, a dipping method, a casting method, an injectionmethod, an ink jet method and the like. The subsequent drying by theheating treatment is performed at 50° C. to 120° C. for 10 to 30minutes. Examples of the irradiation light for the above-mentionedexposure used herein include visible light, ultraviolet light, infraredlight, X-rays, alpha rays, beta rays, gamma rays and the like.Preferably, ultraviolet light is used. This is because the use ofultraviolet light achieves irradiation with large energy to provide ahigh rate of hardening, and an irradiation apparatus therefor is smallin size and inexpensive to achieve the reduction in production costs. Alight source of the ultraviolet light may be, for example, alow-pressure mercury-vapor lamp, a high-pressure mercury-vapor lamp, anultra-high-pressure mercury-vapor lamp and the like. The dose of theultraviolet light is typically 10 to 10000 mJ/cm², preferably 50 to 3000mJ/cm². The subsequent heating treatment is performed at 80° C. to 250°C., preferably at 100° C. to 200° C., for 10 seconds to two hours,preferably for five minutes to one hour.

The formation of the core-forming resin layer 3 a (with reference toFIG. 1( b)) which is the subsequent step is carried out, for example, byforming a photosensitive resin layer in a manner similar to thephotosensitive resin layer in the step of forming the under claddinglayer 2. The material for the formation of the core-forming resin layer3 a (the cores 3) used herein is a material having a refractive indexgreater than that of the materials for the formation of theabove-mentioned under cladding layer 2 and the over cladding layer 8 tobe described later (as shown in FIG. 2( c)). The adjustment of thisrefractive index may be made, for example, by adjusting the selection ofthe types of the materials for the formation of the above-mentionedunder cladding layer 2, the cores 3 and the over cladding layer 8 andthe composition ratio thereof. The thickness of the core-forming resinlayer 3 a is typically in the range of 10 to 100 μm.

In this preferred embodiment, a photosensitive photoresist is used asthe material for the formation of the above-mentioned resist layer 4 tobe formed subsequently (with reference to FIG. 1( b)). The formation ofthe above-mentioned resist layer 4 is carried out by applying theabove-mentioned photoresist onto the core-forming resin layer 3 a andthen drying the photoresist by a heating treatment. The application ofthe above-mentioned photoresist is achieved, for example, by a spincoating method, a dipping method, a casting method, an injection method,an ink jet method and the like. Alternatively, a dry film resist may bebonded The thickness of the above-mentioned resist layer 4 is typicallyin the range of 10 to 30 μm.

The patterning of the above-mentioned core-forming resin layer 3 a andthe resist layer 4 (with reference to FIG.(c)), which is the subsequentstep, is carried out, for example, in a manner to be described below.First, the above-mentioned resist layer 4 is exposed to irradiationlight through an exposure mask formed with an opening patterncorresponding to the pattern of the cores 3 in a manner similar to thestep of forming the above-mentioned under cladding layer 2. Also, theabove-mentioned core-forming resin layer 3 a is exposed to theirradiation light passing through the resist layer 4. Thereafter, aheating treatment is performed. Next, development is performed using adeveloping solution to dissolve away unexposed portions. This causesportions of the core-forming resin layer 3 a remaining on the undercladding layer 2 to be formed into the pattern of the cores 3, andcauses the resist layer 4 to remain on the remaining portions of thecore-forming resin layer 3 a. Thereafter, a heating treatment isperformed to remove the developing solution in the remaining portions ofthe core-forming resin layer 3 a and the developing solution in theresist layer 4. Thus, the above-mentioned remaining portions of thecore-forming resin layer 3 a are formed into the cores 3. The width ofthe cores 3 is typically in the range of 8 to 50 μm

For the patterning of the above-mentioned cores 3 and the like, theabove-mentioned development employs, for example, an immersion method, aspray method, a puddle method and the like. Examples of the developingsolution used herein include an organic solvent, an organic solventcontaining an alkaline aqueous solution, and the like. The developingsolution and conditions for the development are selected as appropriatedepending on the composition of the photosensitive resin composition ofthe core-forming resin layer 3 a and the resist layer 4. The heatingtreatment after the above-mentioned development is typically performedat 80° C. to 120° C. for 10 to 30 minutes.

The above-mentioned thin metal film 5 to be formed subsequently (withreference to FIG. 1( d)) is a metal layer for use as a cathode duringthe electroplating to be performed later (with reference to FIG. 2( b)).Examples of the metal material of the above-mentioned thin metal film 5include chromium, copper and the like. The thickness of theabove-mentioned thin metal film 5 is typically in the range of 600 to2600 Å.

The removal of the above-mentioned resist layer 4 by etching (withreference to FIG. 2( a)), which is the subsequent step, is carried outto leave the thin metal film 5 only in the grooves 6 defined betweenadjacent ones of the cores 3. This is because, if portions of the thinmetal film 5 are left unremoved on the surface of the resist layer 4,the electroplated layers 7 a (are formed on the portions of the thinmetal film 5 lying on the surface of the resist layer 4 byelectroplating to be performed later (with reference to FIG. 2( b)) sothat the electroplated layers 7 a (the electrical interconnect lines 7)lying in the grooves 6 defined between adjacent ones of the cores 3 areshort-circuited. The above-mentioned etching is performed by immersingthe entire structure (the structure shown in FIG. 1( d)) in an etchantto cause the etchant to penetrate through an end surface (an exposedsurface) of the resist layer 4 into the resist layer 4, thereby removingthe resist layer 4. During the above-mentioned etching, the portions ofthe thin metal film 5 lying on the surface of the resist layer 4 to beremoved are removed simultaneously with the removal of the resist layer4 because those portions of the thin metal film 5 are no longersupported by the resist layer 4. Examples of the above-mentioned etchantinclude aqueous sodium hydroxide solutions and the like.

The above-mentioned electroplating (with reference to FIG. 2( b)) afterthe above-mentioned etching is performed by applying a voltage to aplating bath, with the above-mentioned thin metal film 5 used as acathode in the plating bath. This forms the electroplated layers 7 aobtained by the electroplating on the surface of the above-mentionedthin metal film 5. Examples of the metal material of the electroplatedlayers 7 a formed by the above-mentioned electroplating include copper,nickel, gold, tin and the like. Via-filling plating is preferable as theabove-mentioned electroplating. This is because the via-filling platingis suitable for filling the grooves 6 defined between adjacent ones ofthe cores 3 (with reference to FIG. 2( a)) with the electroplated layers7 a. The electroplated layers 7 a (in the above-mentioned grooves 6 mayincompletely fill the grooves 6 or be raised from the top end surface ofthe grooves 6.

The formation of the over cladding layer 8 (with reference to FIG. 2(c)) after the above-mentioned electroplating is carried out in a mannersimilar to the formation of the above-mentioned under cladding layer 2.Specifically, a photosensitive resin layer is formed so as to cover theabove-mentioned cores 3 and the above-mentioned electrical interconnectlines 7. Thereafter, exposure to light and a heating treatment areperformed to form the above-mentioned photosensitive resin layer intothe over cladding layer 8. The thickness of the over cladding layer 8(the photosensitive resin layer) is typically in the range of 10 to 1000μm.

In this manner, the opto-electric hybrid board A is manufactured on thebase 1 (as shown in FIG. 2( c)). As described above, the opto-electrichybrid board A may be used while being provided on the base 1 or be usedafter the base 1 is separated therefrom or removed therefrom by etchingand the like.

In the above-mentioned opto-electric hybrid board A, the opticalwaveguide includes the plurality of cores 3, and the under claddinglayer 2 and over cladding layer 8 which hold the cores 3 therebetweenfrom below and from above. Also, the electrical interconnect lines 7 areformed between adjacent ones of the cores 3 in the optical waveguide. Inother words, the above-mentioned opto-electric hybrid board A has astructure such that the electrical interconnect lines (conductors) 7 areflush with (in the same layer as) the cores (optical interconnect lines)3 in the optical waveguide. Thus, the above-mentioned opto-electrichybrid board A is significantly thinner than the conventionalopto-electric hybrid board B (see FIG. 3) having the two-layerstructure. Further, since the thin metal film 5 is formed on theopposite side surfaces 32 of the cores 3 as stated above, the thin metalfilm 5 may be used to function as a surface for reflecting light beamspropagating through the cores 3. This further ensures the propagation ofthe light beams. Additionally, the electrical interconnect lines 7 areprevented from being short-circuited because the under cladding layer 2,the cores 3 and the over cladding layer 8 disposed around each of theelectrical interconnect lines 7 are insulators constituting theabove-mentioned optical waveguide.

The patterning of the cores 3 and the resist layer 4 is achieved by theexposure to light, the development and the like in the above-mentionedpreferred embodiment. This patterning, however, may be achieved by othermethods, for example by cutting using a rotary blade and the like. Inthis case, the materials for the formation of the cores 3 and the resistlayer 4 are not limited to the photosensitive materials but may bethermosetting resins and the like.

The resist layer 4 and the portions of the thin metal film 5 lying onthe top surface of the resist layer 4 are removed by etching using anetchant in the above-mentioned preferred embodiment. This removal,however, may be achieved by other methods, for example by a physicalmethod such as polishing and the like.

In the above-mentioned preferred embodiment, the formation of the undercladding layer 2 and the over cladding layer 8 uses the photosensitiveresin as the materials thereof, and is achieved by the exposure to lightand the like. However, other materials and other methods may be used. Asan example, the formation of the under cladding layer 2 and the overcladding layer 8 may use a thermosetting resin such as polyimide resinand epoxy resin as the materials thereof, and may be achieved byapplying a varnish prepared by dissolving the thermosetting resin in asolvent and then performing a heating treatment (typically at 300° C. to400° C. for 60 to 180 minutes) to set the varnish or by other methods. Aresin film may be used as the under cladding layer 2 and the overcladding layer 8.

Next, an example of the present invention will be described. It shouldbe noted that the present invention is not limited to the example.

EXAMPLE Material for Formation of Under Cladding Layer and Over CladdingLayer

A material for formation of an under cladding layer and an over claddinglayer was prepared by mixing 35 parts by weight of bisphenoxyethanolfluorene glycidyl ether (component A) represented by the followinggeneral formula (1), 40 parts by weight of3′,4′-epoxycyclohexylmethyl-3,4-epoxycyclohexane carboxylate which is analicyclic epoxy resin (CELLOXIDE 2021P manufactured by Daicel ChemicalIndustries, Ltd.) (Component B), 25 parts by weight of(3′,4′-epoxycyclohexane)methyl-3′,4′-epoxycyclohexyl-carboxylate(CELLOXIDE2081 manufactured by Daicel Chemical Industries, Ltd.) (Component C),and one part by weight of a 50% propione carbonate solution of 4,4′-bis[di(β-hydroxyethoxy)phenylsulfinio]phenyl-sulfide-bis-hexafluoroantimonate(component D).

wherein R₁ to R₆ are hydrogen atoms, and n=1.

Material for Formation of Cores

A material for formation of cores was prepared by dissolving 70 parts byweight of the aforementioned component A, 30 parts by weight of1,3,3-tris{4-[2-(3-oxetanyl)]butoxyphenyl}butane and one part by weightof the aforementioned component D in 28 parts by weight of ethyllactate.

Manufacture of Opto-Electric Hybrid Board

The material for the formation of the above-mentioned under claddinglayer was applied to the surface of a base made of stainless steel(having a thickness of 20 μm) by a spin coating method, and was thendried by a heating treatment at 100° C. for 15 minutes. Then, exposureby the use of irradiation with ultraviolet light at 2000 mJ/cm² wasperformed through a photomask (exposure mask) formed with a desiredopening pattern. Next, a heating treatment was performed at 100° C. for15 minutes to form the under cladding layer. The thickness of this undercladding layer was 25 μm when measured with a contact-type filmthickness meter. The refractive index of this under cladding layer at awavelength of 830 nm was 1.542.

Next, the material for the formation of the above-mentioned cores wasapplied to the surface of the above-mentioned under cladding layer by aspin coating method, and was then dried by a heating treatment at 100°C. for 15 minutes, whereby a core-forming resin layer was formed.Subsequently, a photoresist was applied onto the core-forming resinlayer by a spin coating method, and was then dried by a heatingtreatment, whereby a resist layer (having a thickness of 20 μm) wasformed. Next, a photomask formed with an opening pattern identical inshape with a core pattern was placed over the resist layer. Then,exposure by the use of irradiation with ultraviolet light at 4000 mJ/cm²was performed by a contact exposure method from over the mask.Thereafter, a heating treatment was performed at 120° C. for 30 minutes.Next, development was carried out using an aqueous solution ofγ-butyrolactone to dissolve away unexposed portions of theabove-mentioned resist layer and the core-forming resin layer.Thereafter, a heating treatment was performed at 120° C. for 30 minutes.This formed remaining portions of the core-forming resin layer in apredetermined pattern into the cores. When measured with an SEM(electron microscope), the dimensions of the cores in cross section were50 μm in width×50 μm in height, and a spacing between adjacent ones ofthe cores was 50 μm. The refractive index of the cores at a wavelengthof 830 nm was 1.588.

Next, a thin metal film (having a thickness of 1500 Å) made of an alloyof chromium and copper was formed by sputtering over the under claddinglayer with the resist layer and the cores which were patterned so as tocover the resist layer and the cores. Thereafter, the resist layertogether with the thin metal film formed on the surface of the resistlayer was removed by immersion in an etchant of an aqueous sodiumhydroxide solution. Then, electroplating was performed on the surface ofthe remaining portions of the thin metal film (portions of the thinmetal film on a surface portion of the under cladding layer and on theopposite side surfaces of the cores) to form electroplated layers madeof copper, thereby filling grooves defined between adjacent ones of thecores with the above-mentioned electroplated layers. Then, theelectroplated layers were used to function as electrical interconnectlines.

Then, the over cladding layer was formed so as to cover theabove-mentioned cores and the above-mentioned electrical interconnectlines in a manner similar to the formation of the above-mentioned undercladding layer. The thickness of the over cladding layer was 25 μm whenmeasured with a contact-type film thickness meter. The refractive indexof the over cladding layer at a wavelength of 830 nm was 1.542.

In this manner, an opto-electric hybrid board in which the cores(optical interconnect lines) and the electrical interconnect lines(conductors) were arranged alternately at the same vertical position andin which the under cladding layer and the over cladding layer weredisposed under and over the alternating cores and electricalinterconnect lines was manufactured on the base.

Although a specific form of embodiment of the instant invention has beendescribed above and illustrated in the accompanying drawings in order tobe more clearly understood, the above description is made by way ofexample and not as a limitation to the scope of the instant invention.It is contemplated that various modifications apparent to one ofordinary skill in the art could be made without departing from the scopeof the invention which is to be determined by the following claims.

1. A method of manufacturing an opto-electric hybrid boards comprisingthe steps of: forming a core-forming resin layer on an under claddinglayer; forming a resist layer on the core-forming resin layer; formingthe resist layer and the core-forming resin layer into a predeterminedpattern by patterning to form the core-forming resin layer into cores;forming a thin metal film on the under cladding layer with the resistlayer and the cores patterned so as to cover the resist layer and thecores; removing the resist layer covered with a portion of the thinmetal film together with the portion of the thin metal film;electroplating the remaining portion of the thin metal film lying onside surfaces of the cores patterned to protrude and lying on a portionof the under cladding layer between the cores to fill grooves definedbetween adjacent ones of the cores with plated layers, thereby causingthe plated layers to serve as electrical interconnect lines; and formingan over cladding layer so as to cover the cores and the electricalinterconnect lines.
 2. The method according to claim 1, wherein theunder cladding layer is formed on a metal base.
 3. The method accordingto claim 2, wherein the metal base is a base made of stainless steel. 4.An opto-electric hybrid board, comprising: a plurality of protrudingcores formed in a predetermined pattern on an under cladding layer; athin metal film formed over side surfaces of the protruding cores and asurface portion of the under cladding layer except where the protrudingcores are formed; electrical interconnect lines including electroplatedlayers buried in grooves defined between adjacent ones of the protrudingcores; and an over cladding layer formed so as to cover the protrudingcores and the electrical interconnect lines.